1. Field of the Invention
This invention relates to a surface fastener in which a multiplicity of engaging elements are molded on a substrate sheet by extrusion or injection molding using thermoplastic synthetic resin, and more particularly to a molded surface fastener in which short hooks have both adequate softness and strength and is very durable, while securing a high engaging rate.
2. Description of the Related Art
A molded surface fastener in which a substrate sheet and hooks are simultaneously and integrally molded by extrusion or injection molding using thermoplastic resin is disclosed in, for example, U.S. Pat. No. 4,984,339. Surface fasteners of this type have increasingly been used as fasteners for industrial materials, car or interior ornamentals, daily goods and even sanitary goods including diaper. Consequently, a variety of kinds of surface fasteners different in size and shape have been manufactured to match the use.
As is also understood from the above-mentioned U.S. Patent Specification, in any of the conventional molded type surface fasteners, unlike the woven type, a delicate shape of hooks cannot be obtained for technological difficulties in molding. Yet if hooks in very small size could be molded, only a very low degree of strength can be achieved for the same size as the hooks of monofilament of the conventional woven type surface fastener, making such molded type surface fasteners far from satisfactory for practical use. Further, according to the conventional molded hook structure, the stem is simple in cross-sectional shape and would hence tend to fall flat laterally from its base. As a result, the individual stems would not restore to its original posture after repeated use, thus lowering the rate of engagement with loops of a companion surface fastener. Therefore, in Order to secure adequate strength, it is absolutely necessary to increase the size of the individual hooks, not only making them rigid but also the number of hooks per unit area (density of hooks) being reduced to lower the rate of engagement with the companion loops.
As a solution, a new hook structure which enables a smooth touch, with the stem hardly falling flat, during the engaging and peeling operation likewise the woven type surface fastener and which increases the rate of engagement to secure adequate strength and durability is disclosed in, for example, U.S. Pat. No. 5,131,119. In the molded type surface fastener disclosed in this U.S. Patent, as shown in FIGS. 20 and 21 of the accompanying drawings, each hook 10' has a hook-shape engaging portion 12' extending forwardly from the distal end of a stem 11', which has a rear surface 11a' rising obliquely in a smooth curve from a substrate sheet 15' and a front surface 11b' rising upwardly from the substrate sheet 15', and a reinforcing rib 13' projecting from a side surface of the stem 11', the cross-sectional area of the hook 10' increasing gradually from a tip of the hook-shape engaging portion 12' toward the base of the stem 11'. The reinforcing rib 13' serves to prevent the stem 11' from falling laterally and also to enable to minimize the size of the stem 11' and the hook-shape engaging portion 12' while securing a required degree of engaging strength.
Alternatively, engaging elements of mushroom-shape can be adopted as the engaging elements instead of the hooks. Since the engaging element of this type has a great engaging strength with the companion loop and hence a desired engaging strength can be secured even if its size is minimized, it is suitable for use in which flexibility is required.
However, according to the conventional molded surface fasteners including the one exemplified by the above-mentioned U.S. Patent Specification, all of the hooks are integrally molded and standing from the flat surface of the substrate sheet. In order that the hook is inserted reliably through the companion loop, it is necessary to set a distance L' between the lower surface of the tip of the hook-shape engaging portion 12' and the surface of the substrate sheet 15' at least several times the substantial size (diameter) of a non-illustrated multifilament yarn of the companion loop in, for example, FIG. 20. Consequently, the size of the conventional hook 10' is decided in association with the size of the companion loop; for example, when the hooks having adequate softness and a very small size and suitable for a paper diaper or the like are molded, a necessary minimum distance between the tip of the hook-shape engaging portion 12' and the substrate sheet 15' for allowing the companion loops to enter is necessarily decided.
In other words, if a predetermined engaging strength is to be secured, the height of the hook 10' is naturally decided and thus it cannot be set to a lower value. In addition, if the hooks 10' stand directly from the surface of the substrate sheet 15', whole of the stem 11' is exposed to the surface of the substrate sheet 15' so that a softer touch of the surface fastener surface cannot be achieved. If the hook 10' is reduced in size, whole of the hook-shape engaging portion 12' is depressed so as to be flexed forwardly so that it cannot come into engagement with a companion loop thus considerably lowering the engaging rate of the whole surface fastener.
Meanwhile, in the case of the above-described mushroom-shape engaging element, if it is reduced in size, its stem become thin necessarily. Therefore, if such mushroom-shape engaging element engages in a so-called hanging engagement with the companion loop due to its structure, the engaging element is easy to get damaged between the engaging portion and the stem, and hence it has a low durability. Further, if the stem is made to be thin, the engaging element is easy to get bent by the pressure of the mating surface fastener, and hence the engaging elements can be hardly engaged with the loops.
Further, if the flexibility of the surface fastener is desired to be secured when the size of the hook or mush room shape engaging element is reduced as described above, the thickness of the substrate sheet has to be made thin also. However, if the thickness of the substrate sheet is set to be thin, the substrate tends to be stretched not uniformly or torn when the engaging elements of a finished surface fastener are pulled off molding cavities in a continuous molding, therefore, reliable molding cannot be performed. Even if the molding is accomplished without trouble, the molded substrate sheet becomes wavy and cannot be suitable for practical use.